1. Field of the Invention
This invention pertains to the field of acoustic actuators and the application of special interest therefor is the active noise control of interior spaces.
2. Description of the Related Art
The minimization or outright elimination of structural vibrations and structure-borne sound has numerous advantages. For instance, minimizing cabin noise in an aircraft or duct noise in a building leads to a much higher comfort level for the people inside. In addition, satellite payload launch noise damage mitigation is desirable in order to minimize damage to payload components.
Essentially, there are two means by which to control unwanted sound and vibration. The first method involves adding additional mass, stiffness, or damping to the structure. The first method techniques, all types of passive control, are best suited for applications where the frequency band of the disturbance is above 1 kHz. The second method, i.e., the active control method, is based upon destructive interference of the sound or vibration field. In active control, a sensor/actuator combination, some components of which are typically located on the surface of the vibrating structure, is used to detect and suppress the disturbance. The vibration signal picked up by the sensor is reconfigured and conditioned to drive the actuator in such a manner as to reduce the net effect of the disturbance. In one embodiment, for example, the actuator is driven such that its output field has the same magnitude but opposite phase as the disturbance.
Current state of the art in active vibration and acoustic control systems is that the sensor and electronic sub-systems are more technologically advanced than the actuator components. Control systems have benefitted from faster and cheaper microelectronics available from the computer industry. Likewise, a wide variety of sensors have been developed including optical fibers, piezopolymers, piezocomposites, and acoustic pressure sensors. Sensor selections can now be based on application specific needs. This means that typically, the weakest link in most active control systems is in actuator technology. Although actuator devices for underwater systems have been advanced, the use of such devices in air is difficult because of the impedance load mismatch between the device and the air medium. The specific acoustic impedance of a medium can be defined as density of the medium multiplied by speed of sound in that medium, and the impedance for water is 3700 times the impedance of air. Consequently, the displacements of the in-air actuators must be improved by orders of magnitude when compared to the displacements of in-water actuators.
In systems aiming to control sound penetration or reflections from a surface, a pressing need is for an actuator that exhibits a displacement at least as large as that associated with the noise source over an appreciable area. Additional considerations include thin geometry, low weight, high spatial uniformity, and smooth and well-behaved transfer functions. The actuator must also be physically rugged enough to withstand normal forces and hazardous exposures.
Many active control systems utilize either hydraulics or large, heavy electromagnetic force transducers as the actuator component. These technologies may often be constrained by packaging and weight limitations. In recent years, piezoelectric materials either in the form of piezopolymers, multilayer stacks, or in bender-type configurations have been studied as the actuator components in active control applications. Multilayer stacks are characterized as generating high force/low displacement whereas the flexors exhibit low force/high displacement capabilities.
Currently under development are techniques for the active noise quieting of interior spaces. The conceptual approach to this effort is to develop an acoustic blanket containing multi-unit arrays that may be mounted either on the structure boundaries or hung from the wall in the interior free space. One of the most difficult aspects of this concept is the actuator sub-system. This particular component is the key to the system in that large spatially uniform displacement over a large area and well-behaved acoustic output functions, preferably at low driving voltage, are required while maintaining low weight and a thin profile. A low driving voltage is safer and high voltage amplifiers and related components have higher weight and size. These two needs are difficult to obtain, as they are inconsistent with present technology capabilities.
An object of this invention is an acoustic actuator which includes a plurality of drivers that produce a large displacement (total displacement) exceeding about 20 xcexcm.
Another object of this invention is a thin and/or lightweight acoustic actuator that includes a plurality of drivers.
Another object of this invention is an acoustic actuator that is capable of covering a large surface area.
Another object of this invention is an actuator which can be operated with a low drive voltage from xe2x88x92300 to +300 Volts at frequency in the range of at least 30 to above 500 Hz.
Another object of this invention is a thin and a lightweight actuator that has large displacement exceeding about 20 xcexcm at frequency of about 30-500 Hz.
Another object of this invention is an acoustic actuator with multiple drivers yielding a piston-like motion.
These and other objects of this invention can be attained by an actuator characterized by a stiff face plate, a backing structure spaced from the face plate and at least one driver disposed between the face plate and the backing structure driving the face plate in piston-like fashion, when actuated by driving voltage.